The present invention concerns a single-winding multi-stage scroll expander by which utilizable rotational force is effectively obtained by a wasted high-temperature steam in areas where heating steam is used, such as the Northern part of North America or Northern Europe.
Expanders for driving an electric generator include a turbine-type, a screw-type, a vane-type and a scroll-type.
To drive a relatively low-output generator, the turbine-type is weak against water, the screw-type is difficult to reduce in size and weight, and the vane-type is not sufficiently durable.
The scroll-type is most suitable as a small-sized, light, durable expander, a fact acknowledged among learned engineers.
The simplest scroll expander in structure comprises a single winding. A high-temperature, high-pressure gas such as air or vapor is fed through the center of a wrap. Expansion of the gas in the expander is converted into rotation of a crankshaft to drive an electric generator. Specifically, the scroll expander is an energy converting device transforming heat energy into kinetic energy.
An expander would provide the maximum or 100% energy conversion efficiency if a pressurized gas in an expansion process was converted under constant entropy or reversible adiabatic change.
However, in an actual scroll expander, high-pressure gas heat is partially lost during the expansion process through the wall of the expander due to heat conduction. The maximum conversion efficiency from thermal energy into kinetic energy is actually about 60%.
According to the inventors' experiments, the polytropic index in the expansion process is about 1.1 when compressed air is used as operating fluid. As the adiabatic index of air is 1.4, the difference translates into heat loss.
Therefore, to improve performance of an expander to increase efficiency, it is important how to reduce heat or internal energy loss during the expansion process. However, it is practically difficult for an expansion chamber to be made of complete adiabatic material, thereby limiting improvement in the performance of the expander.